osd: fix bench block size

The command was declared to take 'size' in dumpling, but was trying to
read 'bsize' instead, so it always used the default of 4MiB. Change
the bench command to read 'size', so it matches what existing clients
are sending.

Fixes: #6795
Backport: emperor, dumpling
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>